At present, personal computers have progressed to be compatible with multimedia, and even in laptop personal computers, color display have been commonly available. In said laptop personal computers, STN liquid crystal display and TFT liquid crystal displays are mainly employed. Further, in recent years, large-sized liquid crystal display has been developed and highly advanced improvements of viewing angle characteristics have been demanded. Therefore, an optical compensation film (also called an anisotropic body) having more advanced compensation properties than before is needed.
Since said STN liquid crystal display is comprised of a display element utilizing a birefringent mode, coloration occurs due to a phase difference formed in said liquid crystal. As a result, said display has a major problem in which it is impossible to carry out black-and-white display as well as color display. In order to overcome said problem, a D-STN system (a system utilizing a compensation liquid crystal cell) has been attempted. However, said system runs counter to the time's demand in terms of “thin and light”, featuring the liquid display. In addition, problems have occurred in which high accuracy is demanded for the production of the compensation liquid crystal cell, and the production yield is low.
In order to solve these problems, various types of proposals have been offered. For example, Japanese Patent Publication Open to Public Inspection No. 63-149624 proposes an F-STN system employing a stretched resinous film, and Japanese Patent Publication Open to Public Inspection Nos. 3-87720 and 4-333019 propose a method to carry out color compensation employing a film in which a liquid crystalline polymer is subjected to twisted orientation for the purpose of a decrease in the weight as well as wall thickness of the D-STN system while maintaining its compensation performance. The phase difference compensating plate of this liquid crystal display is comprised of a transparent substrate having thereon an orientation layer and thereon a liquid crystal polymer layer on which a twisted crystal polymer is fixed.
Furthermore, recently, Japanese Patent Publication Open to Public Inspection No. 7-191217 discloses an attempt in which as the compensation of the viewing angle of a TFT-TN display, a discotic liquid crystal film is arranged on the upper and the lower surfaces of a liquid crystal cell, and the characteristics of the viewing angle of said liquid crystal cell is thereby improved. Said compensation film for a TN type liquid crystal display is comprised of an optically isotropic layer in which liquid crystalline compounds are oriented on a resinous film which is nearly optically isotropic, in the same manner as the phase difference compensating plate of the liquid crystal display described in aforementioned Japanese Patent Publication Open to Public Inspection Nos. 3-87720 and 4-333019.
The following three methods as described below have been proposed as an optically compensation sheet for obtaining a wide viewing angle of a liquid crystal display.
(1) a method providing a discotic liquid crystalline compound, which is a negative uniaxial compound, on a support.
(2) a method providing on a support a nematic polymeric liquid crystalline compound with a positive optical anisotropy, which is subjected to hybrid orientation in which the pretilt angle of the liquid crystal molecules varies in the thickness direction.
(3) a method providing on a support two layers containing a nematic liquid crystalline compound with a positive optical anisotropy, in which the orientation direction of the layers crosses each other at approximately 90 degrees, so that an optical property approximate to a negative uniaxial optical property is obtained.
However, the above methods have the following problems.
Method (1) shows a defect specific to a discotic liquid crystalline compound in that, in a TN mode liquid crystal display panel employing the discotic liquid crystalline compound, the displayed image appears yellowish, when viewing the panel obliquely.
In the method (2) a temperature developing a liquid crystal is high and orientation cannot be fixed on an isotropic transparent support such as TAC (cellulose triacetate), and requires additional processing, in which a liquid crystalline compound is oriented and fixed on a first support, and transferred onto a second support such as TAC. This processing is more complex, resulting in lowering of productivity.
There is disclosed in, for example, Japanese Patent O.P.I. Publication No. 8-15681, one example of an optically anisotropic layer employing a positive uniaxial low molecular weight liquid crystalline compound according to method (3). The example is an optically anisotropic layer comprised of four layers which consist of a first oriented layer having an orientation ability, a rod-shaped, positive uniaxial low molecular weight liquid crystalline compound layer, in which the liquid crystalline compound is oriented and fixed, provided on the first oriented layer, a second oriented layer having an orientation ability provided on the rod-shaped, positive uniaxial low molecular weight liquid crystalline compound layer, and a rod-shaped, positive uniaxial low molecular weight liquid crystalline compound layer, in which the liquid crystalline compound is oriented and fixed, provided on the second oriented layer. In this example, a property approximate to a disc-shaped compound can be obtained, for example, by arranging the two rod-shaped, liquid crystalline compound layers so that the orientation directions in the plane of the two layers cross each other at 90 degrees.
Accordingly, method (3) above is extremely advantageous in usage in a liquid crystal television giving priority to color reproduction, since there is no problem of yellowing occurring in the use of a discotic liquid crystalline compound.
Although the use of the discotic liquid crystalline compound requires only one layer, however, the method requires two liquid crystalline compound layers, resulting in lowering of efficiency.
However, the above three methods have, in common, a fundamental problem. That is, in order to obtain optical compensation ability, these methods require an optical compensation sheet to be provided on both sides of, for example, a liquid crystal cell. This means that even the method for employing an optical compensation sheet, which is convenient for improving viewing angle, results in a cost increase. In these methods, the use of one optical compensation sheet destroys symmetry, and results in asymmetry of the viewing angle. For example, when the optical compensation sheet, the rubbing axis of which is rotated 45 degrees, is arranged, symmetry may be improved but the viewing angle property is not improved. There have been no proposals in which the use of only one optical compensation sheet improves the viewing angle property to the same degree as or more than two optical compensation sheets. Further, examples of optical compensation film, which result in improvement of the viewing angle characteristics equal to IPS (lateral electric field mode) as well as VA (perpendicular orientation mode), have not been obtained.